The invention relates generally to strain gauge load cells for weighing and, more particularly, to an improved load cell and system for determining the liquid quantity in cryogenic tanks or cylinders.
Cryogenic liquids stored in tanks are used in a variety of industries. For example, it is known in the restaurant industry to have carbonated beverage dispensing systems wherein gaseous carbon dioxide is mixed with syrup, and sometimes water, to produce a desired carbonated beverage. The syrup, water and carbon dioxide are stored in separate containers with the carbon dioxide stored in liquid form in cylinders or tanks for space efficiency purposes. The liquid carbon dioxide and syrup tanks are periodically refilled by delivery trucks. Examples of such systems may be found in U.S. Pat. Nos. 4,683,921 and 5,234,035, both to Neeser, and U.S. Pat. No. 5,174,354 to Neeser at al.
Clearly it is necessary to check the liquid level of a liquid carbon dioxide tank to determine if the tank need to be replenished. One way to do this is to mount the liquid carbon dioxide tank on a scale. The weight of the tank is subtracted from the weight indicated by the scale to obtain the weight, and thus the quantity, of the liquid carbon dioxide in the tank. Such scales typically include a single load cell centrally positioned between two horizontal and parallel plates that form the framework of the scale. The top plate forms a platform to which the tank legs are bolted. The bottom plate is usually bolted to the floor of the restaurant or other establishment.
A shear beam load cell is often used in a scale for weighing tanks and includes a rectangular beam of load cell quality metal alloy which has one end arranged to be supported on a base in cantilever fashion and a free end arranged to support a vertical load. Strain gauges are mounted to the top and/or bottom of the beam between the supported end and the loaded end. The portion of the beam in the vicinity of the strain gauges may be hollowed out or formed into a web so that the stress on the loaded beam is focused in the area where the strain gauges are mounted. This increases the accuracy of the load cell.
The strain gauges typically consist of thin film or metal foil strain gauge circuits that are mounted to the beam by a glueing process. The basic principle is that a wire or other conductor changes its electrical resistance when deformed. An electric current is passed through the strain gauge circuit. The resistance of the strain gauge changes as the surface to which it is mounted (the top or bottom of the beam) strains or is deformed. Since the strain gauge is bonded throughout its entire length, the gauge is able to sense compressive or tensile strain or deformation of the beam. The resistance change of the strain gauge is proportional to the strain or deformation, as measured by appropriate instruments.
A problem with scales featuring a single load cell in the center is that the mounting surface has to be relatively level for the scale to produce accurate readings. In addition, the mounting of the tank to the top platform and the bottom platform to the floor has to be very rigid or else rocking of the tank upon the single load cell will occur. Such rocking would also adversely effect the accuracy of the scale as well as the stability of the tank mounting.
As an alternative to scale systems that use single load cells, container weighing systems wherein multiple load cells are used have been developed. In such systems, a number of strain gauge load cells are positioned between the legs of the tank and the surface to which the tank is mounted. As a result, the weight of the container and its contents is supported by the load cells. The load cells are connected to summing and calibration circuitry which provides an output to a display from which the weight of the contents of the container may be obtained. Examples of such systems are presented in U.S. Pat. No. 2,980,414 to Perry et al., U.S. Pat. No. 4,044,920 to Swartzendruber and U.S. Pat. No. 4,407,160 to van de Velde. None of the these references, however, illustrate a system for determining the weight of tanks filled with cryogenic liquids.
In addition, the load cells used by prior art weighing systems suffer from a number of disadvantages. More specifically, their complexity makes them expensive to manufacture and may pose maintenance and reliability issues. In addition, prior art load cells feature beams that are supported on the bottom side and support loads on the top side. Such an arrangement makes the prior art load cells difficult to clean under. This can be a significant disadvantage in the restaurant or food service industry.
Accordingly, it is an object of the present invention to provide a load cell and weighing system that can measure the amount of liquid in a tank accurately.
It is another object of the present invention to provide a load cell and weighing system that is easy to install.
It is another object of the present invention to provide a load cell and weighing system that supports a tank in a stable and secure fashion.
It is still another object of the present invention to provide a load cell and weighing system that is economical to manufacture and requires minimal maintenance.
It is still another object of the present invention to provide a load cell that permits cleaning underneath.
The present invention is directed a load cell and a system for determining the quantity of a liquid, such as liquid carbon dioxide, in a tank having a number of legs. A dedicated load cell is position under each leg of the tank. Alternatively, an active load cell may be placed under one leg of the tank with dummy load cells placed under the remaining legs.
Each load cell includes a frame constructed from a metal plate that is folded so that a top panel, sides and ends are formed. The frame is adapted to be positioned on a surface with the top panel spaced from the surface. A beam having a distal end portion and a proximal end portion is suspended by its proximal end portion from the top panel of the frame with a spacer therebetween. The distal end of the beam is provided with a platform support that passes through an opening formed in the top panel of the frame and a platform is mounted on top of the platform support. Each platform is adapted to receive one of the legs of the tank so that a number of the load cells support the tank. A strain gauge circuit is attached to the top surface of each load cell beam between the proximal and distal end portions.
The strain gauge circuits of the load cells communicate with a summing and calibration circuit or a summary board. The summary board processes and combines the signals from the load cells so that the quantity of liquid carbon dioxide in the tank may be determined. The output of the summary board may be directed to a display device and/or a transmitter that is in communication with an antenna so that the quantity of liquid carbon dioxide in the tank may be transmitted to a centralized facility as telemetry. The centralized facility, which may receive telemetry from a number of remote tank locations, dispatches a delivery truck to refill the tank based upon the liquid quantity information received.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.